HVDC (high-voltage direct current) electrical power transmission uses direct current for the transmission of electrical power. This is an alternative to alternating current electrical power transmission which is more common. There are a number of benefits to using HVDC electrical power transmission. HVDC is particularly useful for power transmission over long distances and/or interconnecting alternating current (AC) networks that operate at different frequencies.
A first station may therefore transmit electrical energy to a second station over one or more DC transmission lines, e.g. overhead lines or subsea or buried cables. The first station may generate the DC supply by conversion from a received AC input supply. The second station then typically provides conversion back from DC to AC. Each of the first and second stations may therefore typically comprise a converter for converting from AC to DC or vice versa.
Initially HVDC power transmission systems tended to be implemented for point-to-point transmission, i.e. just from the first station to the second station. Increasingly however it is being proposed to implement HVDC power transmission on a mesh-network or DC grid comprising a plurality of DC transmission paths connecting more than two voltage converters. Such DC networks are useful, for example, in applications such as electrical power generation from renewable sources such as wind farms where there may be a plurality of sources that may be geographically remote.
To date most HVDC transmission systems have been based on line commutated converters (LCCs), for example such as a six-pulse bridge converter using thyristor valves. LCCs use elements such as thyristors that can be turned on by appropriate trigger signals and remain conducting as long as they are forward biased.
Increasingly however voltage source converters (VSCs) are being proposed for use in HVDC transmission. VSCs use switching elements such as insulated-gate bipolar transistors (IGBTs) that can be controllably turned on and turned off independently of any connected AC system. VSCs are thus sometime referred to as self-commutating converters.
Various designs are VSC are known. In one form of known VSC, often referred to as a six pulse bridge, each valve connecting an AC terminal to a DC terminal comprises a set of series connected switching elements, typically IGBTs, each IGBT connected with an antiparallel diode. The IGBTs of the valve are switched together to connect or disconnect the relevant AC and DC terminals, with the valves of a given phase limb (i.e. the two valves that connect the two DC terminals respectively to the same AC terminal) being switched in antiphase. By using a pulse width modulated (PWM) type switching scheme for each arm, conversion between AC and DC voltage can be achieved.
In another known type of VSC, referred to a modular multilevel converter (MMC), each valve comprises a series of cells connected in series, each cell comprising an energy storage element, such as a capacitor, and a switch arrangement that can be controlled so as to either connect the energy storage element in series between the terminals of the cell or bypass the energy storage element. The cells or sub-modules of a valve are controlled to connect or bypass their respective energy storage element at different times so as to vary over the time the voltage difference across the valve. By using a relatively large number of sub-modules and timing the switching appropriately the valve can synthesise a stepped waveform that approximates to a sine wave and which contain low level of harmonic distortion. As will be understood by one skilled in the art there are various designs of MMC. For example an MMC may be a half-bridge MMC or a full bridge MMC. In a half-bridge MMC the energy storage element of a cell or sub-module is connected with a half-bridge switch arrangement, which allows the energy storage element to be bypassed or connected to provide a voltage of a given polarity at the terminals of the cell. In a full-bridge MMC the energy storage element of a cell or sub-module is connected with a full-bridge switch arrangement, which allows the energy storage element to be bypassed or connected to provide a voltage of either polarity at the terminals of the cell.
In normal use the VSCs of the HVDC stations are typically controlled with reference to the AC waveform of the relevant connected AC network to achieve a desired power flow. One VSC may operated in voltage control to control the voltage of the DC lines, with another VSC being controlled in a power control to control power flow.
In use the DC lines are thus charged to the relevant operating DC voltages. On initial start-up of the DC network, or in some instances on re-start after a fault, it can therefore be necessary to charge the DC lines up to the operating voltages.
Before start-up of the DC link, or following some fault conditions, the VSCs connected to the DC network may be in a blocked, non-operational, state. Typically one VSC is used as an energising converter and is de-blocked in voltage control mode and used to charge the DC line(s), with the other converter(s) remaining in the blocked state. The energising converter thus charges a DC line at its proximal end where the DC line may effectively be open-circuited at its distal end. This can result in voltage oscillations in the DC line that can result in a voltage magnitude at the distal end that is greater than 1 p.u. and which may significantly exceed the rated voltage of the DC link, which is undesirable.
The embodiments of the present disclosure provide methods and apparatus which at least mitigate such problems of oscillation.
Thus an embodiment of the present invention provides a method of controlling a voltage source converter to energise a DC link comprising: controlling the voltage source converter to generate a DC voltage on the DC link based on a voltage order; wherein the voltage order is based on a time varying voltage reference signal and the rate of change of the voltage reference signal varies over time.
The method thus controls the VSC which is energising the DC link by generating a voltage order based on a time varying voltage reference signal where the rate of change of the voltage reference signal varies over time. In particular the rate of change of the time varying voltage reference signal may decrease over time. The extent of any voltage oscillations on the DC link will depend on the rate of change of voltage on the DC link and thus on the rate of change of the voltage reference signal used to derive the voltage order. It has been appreciated however that during the first stages of energising the DC link a faster rate of change may be tolerated as the DC link will not yet be up to the full operating voltage and thus even with an oscillation component an over-voltage on the DC link is unlikely. However as the DC voltage increases the rate of change of the voltage may be reduced, i.e. to a more gradual rate of change, so that the magnitude of any voltage oscillation at the distal end is reduced and may, for example, by maintained within an acceptable safe operating limit.
The time varying voltage reference signal may comprise a ramp signal having a slope that varies over time. The ramp signal may ramp from an initial value corresponding to the voltage of the DC link when the voltage source converter is initially de-blocked to a final value corresponding to the nominal voltage of the DC link.
The ramp signal may have a first slope for a first period and a second slope for a second period following the first period, wherein the second slope is decreased, i.e. more gradual, compared to the first slope. The second period may start when the ramp signal reaches a predetermined threshold. There may be at least one additional period subsequent to the second period, wherein the slope of the ramp signal in each period is decreased from the slope in the previous period.
In some embodiments the method may also comprise monitoring DC current flow to determine an indication of current oscillation; and generating the voltage order based on the time varying voltage reference signal and an indication of DC current flow. For example the rate of change of the time varying voltage reference signal could be controlled based on the extent of any determined DC current oscillation. In some embodiments the method may comprise modulating the time varying voltage reference signal by the indication of current oscillation to provide oscillation damping.
Thus the DC current flow may be monitored to determine the extent of any oscillations in current and the time varying voltage reference signal may be modulated based on the indication of current oscillation to provide the voltage order. The voltage order is thus effectively modulated to damp any oscillations in DC current at the VSC, which has the effect of damping any oscillations in voltage of the DC link, as will be explained in more detail later.
The indication of current oscillation may be determined by filtering a signal indicative of DC current flow, e.g. by filtering the DC current flow to identify an oscillation component. Filtering the signal indicative of DC current flow may comprise applying at least one of a band-pass filter and/or a high-pass filter. The pass-band or cut-off frequency of the filter(s) is/are chosen to isolate the oscillation component.
In some embodiments the indication of current oscillation may be compared to a reference current value.
A current controller may receive the indication of current oscillation and determine a damping control signal for modulating the time varying voltage reference signal. In some embodiments the current controller may be a proportional-integral controller, although other types of controller may be used.
In some embodiments the value of the damping control signal may be controlled so as not to exceed a predetermined limit. The predetermined limit may be fixed or, in some embodiments, the predetermined limit may vary over time.
In one aspect there is provided a method of starting an HVDC system comprising a first voltage source converter connected to at least a second voltage source converter by a DC link. The method may comprise: de-blocking the first voltage source converter and controlling the first voltage source converter according to the method as described in any of the variants above whilst maintaining the second voltage source converter in a blocked state; and subsequently de-blocking the second voltage source converter.
In another aspect there is provided machine readable code stored on a non-transitory storage medium, the code comprising instructions for causing a suitable processor to perform the method of controlling a VSC of any of the variants described above.
Aspects also relate to a control apparatus for controlling a voltage source converter to energise a DC link comprising: a voltage order generating module for generating a voltage order for controlling the voltage source converter to generate a DC voltage on the DC link based on a time varying voltage reference signal; and a voltage reference module for generating said time varying voltage reference signal, wherein the rate of change of the voltage reference signal varies over time.
The control apparatus offers all of the same advantages and may be implemented in all of the same variants as discussed with reference to the first aspect of the invention.
In particular the rate of change of the time varying voltage reference signal may decrease over time. The voltage reference module may comprise a ramp generator for generating a ramp signal with a slope that decreases over time. The ramp generator may generate a signal that ramps from an initial value corresponding to the voltage of the DC link when the voltage source converter is initially de-blocked to a final value corresponding to the nominal voltage of the DC link.
The ramp generator may generate a ramp signal having a first slope for a first period and a second slope for a second period following the first period, wherein the second slope is decreased, i.e. more gradual, compared to the first slope. The ramp generator may be configured such that the second period may start when the ramp signal reaches a predetermined threshold. There may be at least one additional period subsequent to the second period, wherein the slope of the ramp signal in each period is decreased from the slope in the previous period.
The voltage order generating module may comprise an oscillation damping module for modulating the voltage reference signal based on the indication of current oscillation.
The oscillation damping module may comprise at least one filter configured to receive and filter a signal indicative of DC current flow. The output of the filter may be the indication of current oscillation. The at least one filter may comprise a band-pass filter and/or a high-pass filter.
The control apparatus may comprise a current controller configured to receive the indication of current oscillation and determine a damping control signal for modulating the time varying voltage reference signal. The current controller may be part of the oscillation damping module. The current controller may comprise a proportional-integral controller, although other types of controller may be used.
In some embodiments the control apparatus may comprise a limiter for limiting the value of the damping control signal so as not to exceed a predetermined limit. The predetermined limit may be fixed or, in some embodiments, the predetermined limit may vary over time.